An available bandwidth at a link is an unused capacity. Its measuring
and/or estimation is not simple in practice. On the other hand, we know
that its continuous knowledge is crucial for the operation of almost all
networks. Therefore, there is a continuous effort in improving the
existing and developing new methods of available bandwidth measurement
and/or estimation. This paper deals with these problems. Network
calculus terminology allows to express an available bandwidth in terms
of a service curve. The service curve is a function representing a
service available for a traffic flow which can be measured/estimated in
a node as well as at an endto- end connection of a network. An Internet
traffic is highly unpredictable what hinders to a large extent an
execution of the tasks mentioned above. This paper draws attention to
pitfalls and difficulties with application of the existing network
calculus methods of an available bandwidth estimation in a real Internet
Service Provider (ISP) network. The results achieved in measurements
have been also confirmed in simulations performed as well as by
mathematical considerations presented here. They give a new perspective
on the outcomes obtained by other authors and on their interpretations.